Isopropylidene derivatives structure

O-Isopropylidene derivatives of carbohydrates form structural isomers from carbohydrates which themselves are epimers. Since structural isomers often fragment differently whereas epimers do not, mass spectra of these derivatives may permit interpretation in terms of stereochemistry. Although molecular-ion peaks are not observed, the molecular weight can be determined readily from a relatively intense M-CH/ peak, resulting from loss of a methyl radical from a 1, 3-dioxolane ring (12). 

The crystalline tri-0-isopropylidene-(2-hydroxy-L-x2/Zo-hexose) prepared from L-gulosone closely resembles the corresponding o-arabino-hexose derivative7 (p. 73) and is believed to have a similar structure.193 On graded hydrolysis, it forms a di-O-isopropylidene derivative which may be characterized as the crystalline diacetate. 

D-Ribonolactone is a convenient source of chiral cyclopentenones, acyclic structures, and oxacyclic systems, useful intermediates for the synthesis of biologically important molecules. Cyclopentenones derived from ribono-lactone have been employed for the synthesis of prostanoids and carbocyclic nucleosides. The cyclopentenone 280 was synthesized (265) from 2,3-0-cyclohexylidene-D-ribono-1,4-lactone (16b) by a threestep synthesis that involves successive periodate oxidation, glycosylation of the lactol with 2-propanol to give 279, and treatment of 279 with lithium dimethyl methyl-phosphonate. The enantiomer of 280 was prepared from D-mannose by converting it to the corresponding lactone, which was selectively protected at HO-2, HO-3 by acetalization. Likewise, the isopropylidene derivative 282 was obtained (266) via the intermediate unsaturated lactone 281, prepared from 16a. Reduction of 281 with di-tert-butoxy lithium aluminum hydride, followed by mesylation, gave 282. 

The di-O-isopropylidene derivatives of D-apiose are prepared by treating the sugar with acetone containing 5% (v/v) of concentrated sulfuric acid.61-63,130 The structures of these derivatives were first... 

Bell114 has suggested that the very ready hydrolysis of a -diisopro-pylidene-D-fructose might possibly indicate a furanose structure, namely, that a-diisopropylidene-D-fructose is l,2 4,6-diisopropylidene-D-fructo-furanose. Hydrolysis would have to be accompanied by change in the size of the ring since the structure of the mono-isopropylidene derivative is, without doubt, pyranose. In this connexion, however, it should be... 

The rules which lead to the predicted structures of column two do not relate to the isopropylidene derivatives but the data of the last column are inserted for convenient comparison. 

Diacetate Derivative, Isopropylidene Formation, and Mass Spectrometry. A particularly facile route to proof of structure of the glyceryl ether components is to use a combination of (a) purification by thin-layer chromatography and (b) separation into individual species of the isopropylidene or acetate derivatives by gas-liquid chromatography. As an example, the experimental protocol for examination of the diacetates will be described at this point and later that of the isopropylidene derivative. Inherent in such an experimental approach is to have synthetic standards of high purity available. It is possible to accomplish this task, but it is not necessarily quick and neat. The synthesis... 

In addition to examination of the diacetates, a particularly effective assay technique involves formation of the isopropylidene derivative as described in Figure 4-22. The product, which is obtained in 95-98% yield, is good proof for the presence of a l-(3)-0-alkylglycerol since this acetonation reaction will occur only with vzc-glycol type structures. Thus this would eliminate substitution of an ether bond at the sn-2 position. The isopropylidene derivative can be analyzed by GC and/or GC-MS these techniques reveal excellent patterns, which are very valuable for proof of structure of the ether as such and for... 

Vitamins of group B were analysed in different forms [530]. Isopropylidene derivatives showed selectivity of the chromatographic separation which was caused by even minor structural differences. Several compounds from the pyridoxine group can be analysed after their conversion into acetates acetylation followed by GC also appeared suitable for three vitamins and 4-pyridoxic lactone. TMS derivatives were recommended for GC separation of the phosphate form of vitamins. When treated with BSTFA—pyridine (1 1) at 60°C for 15 min, biotin provides a completely silylated derivative, which was analysed on a column packed with 3% of OV-17 [531 ]. 

In accordance with the structure XLVIII (R = OMe), O-methylakuammine is reduced by lithium aluminum hydride to O-methyl-akuamminol (LVI R = OMe), an indoline base which contains two alcoholic hydroxyl groups, and which, like pseudoakuammigol (LVI R = H), gives rise to an (amorphous) isopropylidene derivative. 

In the field of natural red algal galactans, MS was used for the first time in the structural analysis of odonthalan.353 Galactose and 6-O-methylgalactose, the hydrolysis products of the polysaccharide, were separated by column chromatography and identified by MS in the form of their di-O-isopropylidene derivatives. Cyclic derivatives of this type had been shown previously to be especially suitable for mass-spectrometric characterization of the structure and (in certain instances) stereochemistry of monosaccharides.349,354 A combination of GLC and MS was also used for identification of methyl 4.6-O-0 -methoxycarbonylethylidene)-D-galactosides (as their trifluoroacetates) in the methanolysis products of Ji-carrageenan from Petrocelis middendorfii,355... 

Finally, Hirst s later contributions to monosaccharide chemistry must not be overlooked. With E. G. V. and Elizabeth Percival, four different trimethyl, 3,4- and 4,5-dimethyl, and the 4-methyl ethers of D-fructose were synthesized from isopropylidene derivatives having well established structures. Certain methyl ethers of D-mannuronic acid and D-glucuronic acid were also prepared, and their periodate oxidation was compared with that of related ethers of D-galacturonic acid. With Elizabeth Percival, he also contributed articles on the methyl ethers of mono- and di-saccharides, and on glycofuranosides from cyclic carbonates, to Methods in Carbohydrate Chemistry, Vol. 2 (1963). 

The situation becomes even more complex on noting that two isomeric di-O-isopropylidene derivatives have been obtained from 1,6-dichloro-l, 6-dideoxy-D-mannitol. These must be structural isomers, and of the three possibilities, 2,4 3,5, 2,5 3,4 and 2,3 4,5, the first is found to be con-formationally very unfavorable, although none of the three may be excluded from consideration. A diacetal could not be obtained " from the reaction of acetone with 1,6-di-O-benzoyl-D-mannitol. 

Mono-isopropylidene derivatives of adenosine and guanosine have also been prepared by Levene and Tipson. They are probably, likewise, the 2,3-isopropylidene derivatives, but their structures have not yet been proved. 

Furthermore Levene and Tipson discovered that uridine will condense with acetone in the presence of sulfuric acid and anhydrous copper sulfate to give a mono-isopropylidene-uridine. On treatment with tosyl chloride it gave a tosyl-isopropylidene-uridine which reacted readily with sodium iodide in acetone to give a crystalline monoiodo-isopropyli-dene-uridine, showing that the tosyl group is at position (5) of the sugar chain. Hence the isopropylidene derivative is 2,3-isopropylidene-uridine with a furanose ring structure, and uridine is D-ribofuranosyl-uracil. 

Although not often exploited, opportunities exist for modifying the structure of 37/-pyrazoles before nitrogen elimination. For example, the 3//-pyrazole 1 can be obtained either by the addition of 2-diazopropane to methyl but-2-ynoate or by the reaction of hydrazine with the isopropylidene derivative of methyl acetoacetate followed by a manganese(IV) oxide oxidation. ... 

Occurrence. D-glycero-D-ido-2-Octulose (66) constitutes the dominant carbohydrate in the fully hydrated leaves of the resurrection plant Craterostigma plan-tagineum, from where it has been extracted.286 The structure was confirmed by NMR analysis of the tri-O-isopropylidene derivative.287 Resurrection plants have the unique ability to be able to survive up to almost complete dehydration and then be rehydrated in a biologically functional state.288 Upon dehydration there is a conversion of 2-octulose into sucrose. The 2-octulose 66 appears to be used as a storage carbohydrate in the leaves, and is mobilized at night.289... 

O. W. Howarth, N. Pozzi, G. Vlahov, and D. Bartels, NMR structural analysis of a tri-O-isopropylidene derivative of D-g/ycero-D-ido-2-octulose, the major sugar found in the resurrection plant Craterostigma plantagineum, Carbohydr. Res., 289 (1996) 137-142. 

Protection of groups during the oxidation steps of the synthesis frequently is obtained by the general reaction of condensation of acetone with two cis-hydroxyl groups, to form the isopropylidene derivatives. These are readily hydrolyzed in water. The di- and monoderivatives of 2-ketogulonic acid and ascorbic acid have the structures shown in [6],... 

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